dotnet/wpf · error · InvalidOperationException

SR.FixedDocumentReadonly

Error message

SR.FixedDocumentReadonly

What it means

FixedTextPointer is a read-only pointer over a fixed document; inserting text into the text run at the pointer is not a supported mutation. InsertTextInRun unconditionally throws InvalidOperationException(SR.FixedDocumentReadonly) regardless of the supplied string (after a null check).

Solutions

1. Do not attempt text edits on fixed documents; convert to an editable representation (e.g. FlowDocument) if editing is required
2. Guard edits with a capability check on the container (fixed documents are immutable) before calling InsertTextInRun
3. Catch InvalidOperationException and surface a read-only message to the user or disable editing UI
4. Perform content changes at the document/package level (XPS package regeneration) instead of via ITextPointer

Example fix

// before
textPointer.InsertTextInRun("new text"); // always throws
// after
bool isReadOnly = textPointer is FixedTextPointer;
if (!isReadOnly) textPointer.InsertTextInRun("new text");

if (pointer is System.Windows.Documents.FixedTextPointer)
    throw new NotSupportedException("Fixed documents are read-only; cannot insert text.");

bool CanEdit(System.Windows.Documents.ITextPointer p) => p is not System.Windows.Documents.FixedTextPointer;

When it happens

Trigger: Calling ITextPointer.InsertTextInRun on any FixedTextPointer, at any position, with any non-null string.

Common situations: Generic text-editor logic (designed for RichTextBox/FlowDocument editing) invoked against a FixedDocument view, e.g. IME input, spell-check correction or programmatic text substitution routed through a TextNavigator over fixed content.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• SR.DocumentReadOnly
• 0x80040209
• args
• Cannot remove signature from read-only file.
• Cannot sign read-only file.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/de8f223978ac24df. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/FixedTextPointer.cs:495

        /// </summary>
        bool ITextPointer.MoveToNextInsertionPosition(LogicalDirection direction)
        {
            return TextPointerBase.MoveToNextInsertionPosition(this, direction);
        }

        //
        // TextContainer modification methods. Disabled by design.
        //
        // This is readonly Text OM. All modification methods returns false
        //

        /// <summary>
        /// </summary>
        void ITextPointer.InsertTextInRun(string textData)
        {
            ArgumentNullException.ThrowIfNull(textData);

            throw new InvalidOperationException(SR.FixedDocumentReadonly);
        }

        /// <summary>
        /// <see cref="ITextPointer.DeleteContentToPosition"/>
        /// </summary>
        void ITextPointer.DeleteContentToPosition(ITextPointer limit)
        {
            throw new InvalidOperationException(SR.FixedDocumentReadonly);
        }

        /// <summary>
        /// <see cref="ITextPointer.ValidateLayout"/>
        /// </summary>
        bool ITextPointer.ValidateLayout()
        {
            return TextPointerBase.ValidateLayout(this, ((ITextPointer)this).TextContainer.TextView);
        }

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